Batteries are welded in series or in parallel to form a battery pack. Along with different assembly manners and different heat-dissipation environments, the larger the number of batteries is, the poorer the temperature uniformity of the batteries is. Therefore, the degradation of the batteries in a certain region of the battery pack is faster than that of the batteries in other regions. If the problem cannot be solved, the degradation of slightly damaged batteries may be accelerated, and the originally perfect battery may be damaged ahead of time due to the influence of the damaged batteries, which is a reason why the lifetime of the battery pack is shorter than that of a battery. Particularly, the risk of damage more easily occurs to the battery pack with a larger capacity.
Therefore, the temperature uniformity of batteries in the battery pack will benefit the lifetime of the battery pack, which is achieved by controlling each of the batteries and by using a cooling device for the purpose of temperature uniformity and heat dissipation. According to the basic theory of heat transfer, there are three manners of temperature control method for the battery, which can be understood from the following heat transfer formula:
      ρ    ⁢                  ⁢          VC      P        ⁢                  ∂        T                    ∂        t              =            -              hA        ⁡                  (                      T            -                          T              ambient                                )                      +                  Q        .            v      
The temperature rising of the battery is caused by the residual energy after subtracting the amount of heat dissipated to the environment from the amount of heat generated.
In the above formula,
  ρ  ⁢          ⁢      VC    P    ⁢            ∂      T              ∂      t      is temperature change rate of a substance in a transient state, and is related to specific heat and mass. The manner of controlling the temperature of the battery includes: immersing the battery into other high specific heat capacity materials, so that the temperature rising is limited since the materials need to absorb a great deal of latent heat during phase change.
In the formula, −hA(T−Tambient) represents external energy loss, and is related to a thermal convection coefficient, a heat-dissipation area, and a difference between the object surface temperature and ambient temperature. Such a battery temperature control manner is based on temperature control through heat dissipation and needs active thermal management, so the cost needs to be increased to fabricate a heat-dissipation system.
{dot over (Q)}v is an overall heat generation rate, corresponding to a manner of controlling the amount of heat generated to indirectly control the temperature.